1. Field of the Invention
The invention relates to an EGR control device and method for an internal combustion engine.
2. Description of the Related Art
There has been known an EGR unit (an exhaust gas recirculation unit) that recirculates part of exhaust gas of an internal combustion engine into an intake passage. Such an EGR unit, which is designed to reduce an amount of nitrogen oxides (NOx) and the like that are discharged from an engine, is provided with an EGR gas passage through which an exhaust passage and an intake passage of the engine communicate with each other, and with an EGR control valve across which the EGR passage extends. By controlling an opening of the EGR control valve, a flow rate of EGR gas is controlled. Thus, an EGR ratio, which is a ratio of a flow rate of EGR gas sucked by the engine to a flow rate of gas sucked by the engine (cylinders) (i.e., a flow rate of the entire gas), is controlled.
An amount of NOx discharged from a diesel engine is closely correlated to a concentration of oxygen contained in gas flowing into cylinders of the engine (hereinafter referred to as an “intake-air oxygen concentration “x””). Accordingly, in order to reduce a discharge amount of NOx, it is effective to perform control such that the intake-air oxygen concentration “x” becomes equal to a predetermined concentration. In view of the foregoing, an internal combustion engine equipped with an EGR unit disclosed in Japanese Patent Application No. 10-141147 adopts a concept of converted EGR ratio so as to control an intake-air oxygen concentration “x”, and controls an EGR control valve such that an actual converted EGR ratio becomes equal to a target converted EGR ratio.
That is, in the unit disclosed in Japanese Patent Application Laid-Open No. 10-141147 mentioned above, an air excessiveness ratio λ is defined as expressed by an expression (1) shown below. A converted EGR ratio SR is defined as a value obtained by dividing an EGR ratio R by the air excessiveness ratio λ (SR=R/λ). As is apparent from FIG. 5, there is established a relationship approximately expressed by an expression (2) shown below between the converted EGR ratio SR and the intake-air oxygen concentration “x”.
[Expression 1]λ=k×Gn/Q  (1)
k . . . constant
Gn . . . flow rate of air (fresh air) that is newly sucked into engine (cylinders)
Q . . . fuel injection amount per unit time
[Expression 2]SR=R/λ≈p×x+q  (2)
p: negative constant, q: positive constant
Thus, as is understood from the aforementioned expression (2), the intake-air oxygen concentration “x” can be made equal to a suitable concentration by determining a target converted EGR ratio SRtgt as a target value of the converted EGR ratio SR on the basis of operational state quantities (e.g., a command injection amount Qfin and an engine rotational speed NE), calculating a true converted EGR ratio (an actual converted EGR ratio) SRact on the basis of the operational state quantities of the engine, and controlling an opening of the EGR control valve such that the actual converted EGR ratio SRact coincides with the target converted EGR ratio SRtgt. The aforementioned unit of the related art controls EGR ratio on the basis of the concept as described above, thus reducing a discharge amount of NOx.
In this case, the target converted EGR ratio SRtgt is determined, for example, on the basis of a map (a table) defining a relationship between command injection amount Qfin and engine operational state quantity as engine rotational speed NE on one hand and target converted EGR ratio SRtgt on the other hand, and on the basis of an actual command injection amount Qfin and an actual engine rotational speed NE. Further, an actual converted EGR ratio SRact is calculated on the basis of expressions (3) and (4) shown below.
In the expressions (3) and (4), Ract represents an actual EGR ratio, and Gn represents a flow rate of air (fresh air) that is actually sucked into the cylinders of the engine (hereinafter referred to as an “actual air flow rate Gn” or a “detected air flow rate Gn”). The flow rate of air (fresh air) is detected by an air flow meter. Further, Gcyl represents a flow rate of gas that is actually sucked into the cylinders of the engine (i.e., flow rate of the entire gas=flow rate of fresh air+flow rate of EGR gas). This flow rate (hereinafter referred to as a “cylinder inflow gas flow rate Gcyl”) is determined on the basis of a map defining a relationship between intake pipe pressure PM and intake air temperature THA on one hand and gas flow rate Gcyl on the other hand, and on the basis of an actually detected intake pipe pressure PM and an actually detected intake air temperature THA.
Further, an air excessiveness ratio λ used in the expression (3) is calculated on the basis of the aforementioned expression (1). In this case, a fuel injection amount Q per unit time in the expression (1) cannot be measured directly and thus is calculated, for example, from an engine rotational speed NE and a command injection amount (a required injection amount) Qfin, which is determined on the basis of an accelerator operation amount Accp and the engine rotational speed NE.
[Expression 3]SRact=Rac/λ  (3)
[Expression 4]Ract=(Gcyl−Gn)/Gcyl  (4)
However, even if a drive signal has been delivered to an injector so that the command injection amount Qfin of fuel is injected therefrom, an actual fuel injection amount may not coincide with the command injection amount Qfin owing to differences among individual products of the injector or changes in performance of the injector during use thereof. Hence, the fuel injection amount Q per unit time in the aforementioned expression (1) may become imprecise, so that the air excessiveness ratio λ may become imprecise. Therefore, an actual converted EGR ratio SRact, which is based on the aforementioned expression (3), may not be calculated precisely. As a result, the true converted EGR ratio SRact does not coincide with the target converted EGR ratio SRtgt, and the intake-air oxygen concentration “x” cannot be made equal to a desired value “x”. In some cases, therefore, an increase in discharge amount of NOx is caused.